Permanent magnet (PM) motors appear to be gaining momentum in industry. This is due to their increased efficiency and small size, which owes to the presence of a permanent magnet in the rotor. In particular, PM motors with sinusoidal back-EMF are of interest because they produce much less torque ripple than their trapezoidal back-EMF equivalents. Synchronous motors have generally included a position sensor to support position-based control algorithms. Given the apparent increasing market demand for this type of motor, the use of position sensorless technology for their control and drive implementation becomes highly valuable. In fact, the cost reduction associated with removing the position sensor can constitute a significant source of profit in high volume production. Furthermore, since a mechanical position sensor can be bulky and prone to failure in harsh environments, replacing it with a sensorless algorithm can increase the reliability of the motor drive.
The sensorless algorithms that have been proposed in the literature can be classified under two distinct categories, namely, those that work only at high speed and those that work only at low speed. As of today, there are no sensorless algorithms that single-handedly permit operation over the entire speed range. This fact restricts the use of sensorless technology in servo systems, such as those employed in robotics and automotive applications.
The high speed sensorless algorithms employed for PM motors are all directly or indirectly based on extracting position information from the motor back-EMF. Since the back-EMF is practically non-existent at low speed, these techniques cannot operate in the low speed range. As a means for extracting position information from the back-EMF, various algorithms have been proposed. For example, state observers, Kalman filters, hypothetical rotor position, and voltage and current measurement, have been successfully used as high speed sensorless approaches.
The low speed sensorless algorithms are all based on the extraction of position information from a stator inductance variation caused by rotor saliency. Therefore, unlike high speed sensorless algorithms, low speed sensorless algorithms are restricted to operation with PM motors that exhibit saliency. Such saliency is often a characteristic of the construction of the motor, as is the case of an IPM (interior permanent magnet) motor. In an IPM motor, the stator inductance variation caused by rotor saliency can be detected even at low or zero speed, by resorting to various kinds of excitation. The detected stator inductance variation can thereafter be used to extract position information. Hence, the low speed sensorless algorithms can be classified as per their method of excitation and detection of the stator inductance variation. For example, methods based on excitation by test pulses and current amplitude measurement, fluctuating vector excitation and high-frequency impedance measurement, fluctuating vector excitation and phase measurement, and rotating vector excitation and current demodulation, have all been successfully employed for estimating position at low and zero speed. Because of necessary assumptions that become invalid at high speed, these low speed sensorless algorithms cannot operate at high speed. Furthermore, they all share a limitation, whereby the polarity (i.e. positive or negative) of the estimated position is unknown and must be determined by other means.